1. ORIGIN OF ORGANIC MATTER

Organic matter is an important part of the soil and we know from Lecture 1 that mineral soils (sandy, loam and clay soils) contain about 5% organic matter. Peat soils can contain up to 80% of organic matter.

Organic matter: is produced by the decay of dead plants and animals in the soil. In veld and pastures, it comes from dead roots, leaves and stalks of grasses which fall onto the soil and decay. It is also produced from the bodies of dead ants, earthworms, bacteria and the dung of both wild game and grazing animals. Flocks of sheep produce a lot of dung and thus a lot of organic matter. In cultivated soils, organic matter can be added to the soil by ploughing‐in weeds, crop residues such as maize stalks, green crops like legumes, and by adding dung in the form of kraal manure, poultry or pig manure to the soil. Artificial fertilizers such as Ammonium Nitrate or any compound fertilizer help indirectly by increasing plant growth and so producing more crop residues to plough back into the soil.

Because organic matter always contains carbon, it burns easily (coal is pure organic matter) and can be measured in a soil by burning. You can remember from Lecture 2 that the Sand, Silt and Clay fractions of a soil can be measured by a complicated process called Mechanical Analysis. The organic matter fraction can be measured quite simply by the following method which you might like to try  for yourself.

MEASUREMENT OF ORGANIC MATTER

  • Weigh out 20g of dry soil and place in a small heat proof container ‐ e.g. small tin, tin lid or crucible. Call this weight A.
    • Place it over a source of heat, a Bunsen burner or candle flame, and allow to heat until the sample has stopped smoking.
    • Allow for cooling and weigh the remains of the soil; call this weight B. The amount of organic matter in that soil sample will be:

Weight A – Weight B            x 100 Weight A

This will give you the amount of organic matter expressed as a percentage, i.e. 3,5%.

Try the experiment with an ordinary mineral soil (sandy or clay soil) and also with a peat soil which you can obtain from any vlei.

Figure 1: Experiment for measuring organic matter in the soil

FLAME

BREAKDOWN OF ORGANIC MATTER

Organic Matter is broken down from raw plant material to a black substance called humus by means of micro‐organisms which live in the soil. These micro‐organisms use the dead plant materials which fall into the soil for food and they produce Carbon Dioxide, Water and Energy which they use for growth and reproduction. The full explanation of this breakdown will be given in Lecture 6.

  • SOIL MICROFLORA

There are literally millions of soil micro‐organisms living in each cubic centimeter of soil and they are divided into two broad categories:

  • Parasites: those that live and feed on other organisms.
    • Saprophytes: those that live on dead organic matter.

We are concerned with the Saprophytes, and the main soil micro‐organisms in this group are the following:

BACTERIA

  • These are single‐celled (unicellular) plant organisms about 1/1000th of a millimeter in size. They can be seen only under a powerful microscope.
  • They do not contain chlorophyll, the green substance which gives plants their colour.
  • They can multiply very rapidly by a process called binary fission, which means they simply divide into two halves and each half grows to become a full sized cell. Under good conditions, they can do this every half hour producing millions of full sized bacteria in 12 hours.
  • There are many different types of bacteria which are identified by size, colour, shape (when seen under the microscope) and their patterns of growth can be identified when they are grown on Agar Plates. Agar is a jelly‐like substance which is placed in a shallow dish and which sets hard. A drop of liquid containing bacteria is placed on the Agar and the dish is placed in an oven at about 30°C. After two days the bacteria have multiplied and form a characteristic pattern on the Agar.

Examples of Bacteria are:

  • Staphlococcus: one of the bacteria that causes mastitis in dairy
    • Streptococcus: cause the yellow pus in boils.
    • Lactobacillus: found in milk.
    • Salmonella: causes severe diarrhea in animals and humans.
    • Spirochaetes: causes venereal disease in humans
    • Rhizobium: found in the root nodules of legumes and they supply nitrogen from the air to  the plant.
    • Nitrobacter, Nitrosomas: these bacteria are needed to carry out the Nitrogen cycle: see Lesson 6.

All bacteria are either aerobic, requiring Oxygen from the air to survive, or they are: anaerobic and live without Oxygen; these bacteria are found in waterlogged conditions in the soil.

The bacteria that break down Organic Matter thrive in the following conditions in the soil in which they are working:

  • Slightly acidic conditions: a pH of 5,5 to 6,5. They cannot function in very acid conditions such as an acid peat soil.
    • Plenty of soil moisture such as occurs during the rainy season, but not a waterlogged soil as in a vlei.
    • Plenty of air and so Oxygen. The most rapid breakdown of organic matter takes place in moist sandy soils and also in well cultivated soils. The slowest breakdown in heavy, wet clay soils.
    • A temperature of around 30⁰C. Bacterial activity is reduced during the winter.
    • Plenty of food such as plant and animal residues.
    • Some minerals, especially Phosphate. A lack of phosphate can limit bacterial activity in some soils.

During dry spells’, bacteria form spores by thickening their cell wall. They become dormant and cease all activity; in this state, they can survive even boiling Water. When conditions improve they re‐ activate and start multiplying.

FUNGI

These are‐made up of many cells (multi cellular), which are joined together to make up a network of threads called mycelia. They contain no Chlorophyll.

They reproduce by producing spores which spread and develop into new individuals. Common examples of Fungi are:

  • Moulds: these are seen on jam, fruit, etc.
    • Penicillin: used for treating wounds, diseases, mastitis.
    • Mushrooms: these are produced from underground Mycelia.
    • Fusarium: causes Ear Rot in maize and Damping Off in seedlings.
    • Mildew: occurs in plants. Potato Blight.

The Fungi found in soils prefer acidic conditions together with plenty of Air and Moisture. They occur most frequently in sour soils.

Fungi decompose coarse organic matter and also living plants, so they are both Saprophytic and Parasitic.

MYCORRHIZA

These are multi cellular and contain no chlorophyll. Some can fix Nitrogen from the atmosphere. They occur mainly in or on very moist soils. An example of a Mycorrhiza is an Algae which is the  green slime that you see on stagnant water.